Product for a welded construction made of AlMgMn alloy having improved mechanical strength

ABSTRACT

Rolled or extruded products for welded constructions made of AlMgMn type aluminum alloy. These products contain, in % by weight, 3.0&lt;Mg&lt;5.0, 0.75&lt;Mn&lt;1.0, Fe&lt;0.25, Si&lt;0.25, 0.02&lt;Zn&lt;0.40, optionally one or more of the elements Cr, Cu, Ti, Zr such that Cr&lt;0.25, Cu&lt;0.20, Ti&lt;0.20, Zr&lt;0.20, other elements &lt;0.05 each and &lt;0.15 in total, wherein Mn+2Zn&gt;0.75. In the welded state, these products have improved mechanical strength and resistance to fatigue without unfavorable consequences with regard to toughness and corrosion resistance, and are particularly suitable for naval construction, for industrial vehicles and for bicycle frames made of welded tubes.

[0001] This application is a continuation-in-part of U.S. application Ser. No. 08/875,113, filed Jul. 25, 1997, which was filed under 35 USC 371 on the basis of PCT/FR96/00279, filed Feb. 21, 1996.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to the sphere of rolled or extruded products such as sheets, profiles, wires or tubes made of AlMgMn-type aluminum alloy containing more than 3% by weight of Mg, intended for welded constructions having a high yield stress, good resistance to fatigue and good toughness for structural applications such as ships, industrial vehicles or welded bicycle frames.

[0004] 2. Description of Related Art

[0005] The optimum dimensioning of welded structures made of aluminum alloy leads to the use of 5000 series AlMg alloys according to the Aluminum Association nomenclature, in the cold-worked temper (temper H1 according to the standard NF-EN-515) or partially softened temper (temper H2), or stabilized temper (temper H3), while maintaining high resistance to corrosion (temper H116) rather than the annealed temper (temper O). However, the improvement in the mechanical characteristics relative to the temper O does not usually remain after welding, and certifying and monitoring organizations generally recommend that only the characteristics in temper O be taken into consideration for welded structures. The resistance to fatigue and the fissure propagation rate should also be taken into consideration for dimensioning.

[0006] In this sphere, research has concentrated mainly on the implementation of the welding operation itself. There have also been attempts to improve the corrosion resistance of the article by appropriate thermomechanical treatments.

[0007] Japanese patent application JP 06-212373 proposes the use of an alloy containing 1.0 to 2.0% of Mn, 3.0 to 6.0% of Mg and less than 0.15% of iron to minimize the reduction in the mechanical strength due to welding. However, the use of an alloy having such a high manganese content leads to a reduction in the resistance to fatigue and in the toughness.

SUMMARY OF THE INVENTION

[0008] The object of the invention is significantly to improve the mechanical strength and fatigue resistance of welded structures made of AlMgMn alloy, under predetermined welding conditions, without unfavorable consequences for other parameters such as toughness, corrosion resistance and cutting deformation, due to internal stresses.

[0009] The invention relates to products for welded constructions made of an AlMgMn aluminum alloy containing, in % by weight:

[0010] 3.0<Mg<5.0

[0011] 0.5<Mn<1.0

[0012] 0.02<Zn<0.40

[0013] Fe<0.25

[0014] Si<0.25

[0015] optionally one or more of the elements Cr, Cu, Ti, Zr such that:

[0016] Cr<0.25

[0017] Cu<0.20

[0018] Ti<0.20

[0019] Zr<0.20

[0020] other elements <0.05 each and <0.15 in total, wherein Mn+2Zn>0.75.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Contrary to earlier research which concentrated on the welding process and the thermomechanical treatments, the inventors have found a particular, range of composition for minor alloying elements, in particular iron, manganese and zinc, leading to an interesting set of properties combining static mechanical characteristics, toughness, resistance to fatigue, resistance to corrosion and cutting deformation, this set of properties being particularly well adapted to the use of these alloys for naval construction, utility vehicles or the welded frames of bicycles.

[0022] This set of properties is obtained by combining a low iron content, <0.25%, preferably <0.20%, and even 0.15%, and a manganese and zinc content such that Mn+2Zn>0.75%, preferably >0.8%. The Mn content should be >0.5%, preferably >0.8%, to have adequate mechanical characteristics, but should not exceed 1% if a deterioration in toughness and fatigue resistance are to be avoided. The addition of zinc combined with manganese has been found to have a beneficial effect on the mechanical characteristics of welded sheets and joints. However, it is better not to exceed 0.4% because problems can then be encountered in welding.

[0023] The magnesium is preferably kept >4.3%, because it has a favorable effect on the yield stress and fatigue resistance, but beyond 5% the corrosion resistance is less good. The addition of Cu and Cr are also favorable to the yield stress, but Cr is preferably kept <0.15% to maintain good resistance to fatigue.

[0024] The mechanical strength of the sheets depends both on the magnesium content in solid solution and on the manganese dispersoids. It has been found that the volumetric fraction of these dispersoids, which is linked to the iron and manganese contents, should preferably be kept above 1.2%. This volumetric fraction is calculated from the average of the surface fractions measured on polished cuts produced in three directions (length, width and thickness) by scanning electron microscopy and image analysis.

[0025] The products according to the invention can be rolled or extruded products such as hot- or cold-rolled sheets, wires, profiles or extruded and optionally drawn tubes.

[0026] The sheets according to the invention, which are assembled by butt welding by a MIG or TIG process and with a bevel of the order of 45° over about ⅔ of the thickness have, in the welded region, a yield stress R_(0.2) which can be at least 25 MPa higher than that of a conventional alloy having the same magnesium content, that is a gain of about 20%.

[0027] The width of the thermally affected region is reduced by about one third relative to a conventional 5083 alloy, and the hardness of the welded joint increases from about 75 Hv to more than 80 Hv. The welded joints also have a tensile strength exceeding the minimum imposed by organizations monitoring unwelded cold-worked crude sheets.

[0028] The sheets according to the invention have fatigue resistance, measured by plane bending with a stress ratio wherein R=0.1 on samples taken in the cross-longitudinal direction, higher than:

[0029] 10⁵ cycles with a maximum stress >280 MPa;

[0030] 10⁶ cycles with a maximum stress >220 MPa;

[0031] 10⁷ cycles with a maximum stress >200 MPa.

[0032] The fissure propagation rate ΔK, measured when R=0.1, is >22 Mpa✓m when da/dN=5×10⁻⁴ mm/cycle and >26 Mpa✓m when da/dN=10⁻³ mm/cycle.

[0033] The sheets according to the invention usually have a thickness greater than 1.5 mm. With thicknesses greater than 2.5 mm they can be obtained directly by hot rolling, without the need for subsequent cold rolling and, furthermore, these hot-rolled sheets are less distorted on cutting than cold-rolled sheets.

[0034] The products according to the invention have corrosion resistance which is as good as that of normal alloys having the same magnesium content, for example 5083 of common composition, widely used in naval construction.

EXAMPLE

[0035] Eight samples of sheets were prepared by conventional semi-continuous casting in the form of plates, were heated for 20 h at a temperature >500° C. and were then cold-rolled to the final thickness of 6 mm. The reference 0 corresponds to a conventional 5083 composition and reference 1 to a composition slightly outside the invention. The others have a composition according to the invention.

[0036] The compositions were as follows (% by weight): Ref. Mg Cu Mn Fe Cr Zn Ti Zr 0 4.40 <.01 0.50 0.27 0.09 0.01 0.01 1 4.68 <0.01 0.72 0.12 0.05 <0.01 0.01 2 4.60 <0.01 0.85 0.17 0.10 0.16 0.01 3 4.62 <0.01 0.96 0.10 0.05 0.02 0.01 4 4.80 0.09 0.80 0.11 0.03 0.02 0.01 5 4.72 <0.01 0.87 0.13 0.03 0.02 0.01 0.11 6 4.92 0.06 0.94 0.08 0.02 0.19 0.01 7 4.69 <0.01 0.72 0.07 0.02 0.10 0.01

[0037] The samples all have, after rolling, a yield stress R_(0.2)>220 Mpa in the L direction.

[0038] The mechanical strength of the joints welded from these sheets was measured under the following conditions: continuous automatic MIG butt welding with a symmetrical bevel having an inclination of 45° to the vertical over a thickness of 4 mm and filler wire of 5183 alloy.

[0039] The mechanical characteristics (tensile strength R_(m), yield stress R_(0.2)) were obtained by pulling over samples standardized by the Norwegian monitoring organization DNV for naval construction having a length of 140 mm and a width of 35 mm, the weld bead with a width of 15 mm being in the center and the length of the narrow portion of the sample being 27 mm, that is the sum of the width of the bead and twice the thickness (15+22 mm).

[0040] The volumetric fractions of manganese dispersoids was also measured.

[0041] The results are as follows (in MPa for resistances and % for fractions): Ref. R_(m) R_(0.2) Fractions 0 285 131 0.62 1 292 144 1.2 2 300 146 1.6 3 310 158 1.7 4 309 149 1.4 5 305 155 1.5 6 318 164 1.9 7 310 153 1.5

[0042] It is found that the yield stress of samples welded according to the invention increases by between 15 and 35 MPa relative to the reference sample.

[0043] The resistance to fatigue of unwelded sheets subjected to plane bending wherein R=0.1 was also measured for references 0 to 4, while determining the maximum stress (in MPa) corresponding to 10⁶ and 10⁷ cycles respectively, as well as the fissure propagation rate ΔK measured when da/dn=5×10⁻⁴ mm/cycle (in Mpa✓m).

[0044] The results were as follows: Ref. 10⁶ cycles 10⁷ cycles ΔK 0 220 200 22 1 235 205 22 2 225 200 23 3 230 205 22 4 225 200 22

[0045] It is found that, despite the increase in the mechanical strength, the sheets according to the invention have resistance to fatigue which is at least as good as that of conventional 5083 sheets. 

What is claimed is:
 1. Aluminum alloy sheet for a welded construction, consisting essentially of, in % by weight: 3.0<Mg<5.0 0.5<Mn<1.0 0.02<Zn<0.40 Fe<0.25 Si<0.25 Cr<0.25 Cu<0.20 Ti<0.20 Zr<0.20 other elements <0.05 each and <0.15 total, remainder Al, wherein Mn+2Zn>0.75.
 2. Sheet according to claim 1, wherein Mn+2Zn>0.8.
 3. Sheet according to claim 1, wherein Cr<0.15.
 4. Sheet according to claim 1, wherein Mg>4.3.
 5. Sheet according to claim 4, wherein Mg>4.6.
 6. Sheet according to claim 1, wherein 0.10≦Zn<0.40.
 7. Sheet according to claim 1, wherein Mn>0.8.
 8. Sheet according to claim 1, wherein Fe<0.20.
 9. Sheet according to claim 8, wherein Fe<0.15.
 10. Sheet according to claim 1, wherein dispersoids are present in a volumetric fraction greater than 1.2%.
 11. Sheet according to claim 1, having a thickness >2.5 mm and obtained directly by hot rolling.
 12. Sheet according to claim 1, having, in an unwelded state, fatigue resistance, measured by plane bending wherein R=0.1 in the cross-longitudinal direction, higher than: 10⁵ cycles, with a maximum stress >280 MPa; 10⁶ cycles with a maximum stress >220 MPa; 10⁷ cycles with a maximum stress >200 MPa.
 13. Sheet according to claim 1, having a fissure propagation rate ΔK, measured when R=0.1, higher than: 22 Mpa✓m when da/dn=5×10⁻⁴ mm/cycle; 26 Mpa✓m when da/dn=10⁻³ mm/cycle.
 14. Sheet according to claim 1, welded by fusion to form a welded zone, and having a hardness >80 Hv in the welded zone.
 15. Sheet according to claim 1, having a yield stress, measured on a standard DNV sample across an MIG butt welded joint, greater than 140 MPa.
 16. Sheet according to claim 15, having a yield stress, measured on a standard DNV sample across an MIG butt welded joint, greater than 150 MPa. 